dulles has economy lots. Thry could be full.

a few other parking options…

1. do a hotel stay, park and fly if your flight is in the morning
2. park at a metro station for overnight parking. Innovation station, has parking garages as do a few of the stations in the other side of the airport. Ashburn station has a parking garage that is not part of metro parking management you can also use.

before you get to the airport there are color named economy lots that you take a shuttle bus. I have no idea if the lots will get filled. https://airportparkingguides.com/dul...t-parking-map/